1. Field of the Invention
This invention relates to zoom lenses suited to single-lens reflex cameras or video cameras and, more particularly, to a relatively wide-angle and high-range zoom lens having a leading lens unit of negative refractive power, i.e., a so-called negative-lead-type zoom lens having five lens units in total.
2. Description of the Related Art
The negative-lead-type zoom lens has the advantage of increasing the maximum of the angular field to a relatively high value with ease and decreasing the minimum object distance at which close-up photography can be performed. However, on the other hand, it has drawbacks in that the aperture opening needs to be large in diameter and the range of variation of the focal length is difficult to extend.
To achieve a minimization of the size of the entirety of the lens system and a great extension of the range of focal lengths by improving those drawbacks, proposals have been made in, for example, Japanese Patent Publication No. Sho 49-23912 and Japanese Laid-Open Patent Applications Nos. Sho 53-34539, Sho 57-163213 and Sho 58-4113.
In every one of these cited examples, the zoom lens is constructed from lens units of negative, positive, negative and positive refractive powers in this order from the object side, totaling four lens units. Of these, certain lens units are made to move in proper relation when zooming.
Recently, there has arisen a demand for an increase of the zoom ratio with inclusion of wide angles of field coverage with respect to the standard zoom lens for the single-lens reflex camera or video camera. For example, a focal length range of from 35 to 70 mm in the 35 mm format and an even wider range of from 28 to 70 mm are already employed in the standard zoom lenses for the single-lens reflex camera.
More recently, a much desired extension of the range toward the telephoto side with increase of the zoom ratio to a range of from 28 to 80 mm, or from 28 to 85 mm is attempted in the standard zoom lenses.
If such a wide angular field of coverage and such a high zoom ratio are attained, however, a problem arises in that the lens gets a long total length. Another problem arises in that the relation in which the lens components move to vary the focal length becomes complicated. As a result, the lens barrel structure has to be in the form of an increased number of layers. This leads to an increase in the bulk and size of the lens barrel and the complexity of structure of the operating mechanism.